In both the DAP4 and the new larval DAP2 Data Request Forms generated using an XSLT (operating on the DMR and DDX respectively) the implementation of the UI for subsetting Sequence variables is broken. In particular the "select" lis of comparison operators does not work, but more than this may be at issue.
Here's an example:
http://test.opendap.org/opendap/data/ff/1998.6.avhrr.dat.dmr